For the deposition of a thin film of a metal or alloy by methods such as sputtering and evaporation, the sheet resistance of a thin film to be deposited is managed. With conventional arts, after a thin film is deposited, the conditions for depositing the next film are determined by measuring the sheet resistance of a substrate removed from a thin-film depositing apparatus. Alternatively, the conditions for depositing a film are determined by placing on a substrate holder together with the substrate, a sheet-resistance monitoring substrate for monitoring the sheet resistance of a sheet being deposited and by measuring the resistance of the monitoring substrate.
An example of a known conventional sheet-resistance measuring method is a four point probe method. The theory of this method will be explained with reference to FIG. 18. First, four needle-like electrodes (probes) 52 are arranged linearly on a metal film 51 subjected to a sheet resistance measurement, a potential difference V between the two inner probes 52 produced by a current flowing through the two outer probes 52 is measured, and the resistance (V/I) is calculated. Thereafter, a resistivity (.rho.) is calculated by multiplying the above calculated resistance (R) by a correction factor (F) as a dimensionless value determined by the shape and size of the metal film 51 and the positions of the probes 52. However, the four point probe method suffers from such problems that the metal film 51 is scratched and dust is produced because the probes 52 are brought into contact with the metal film 51 by pressures. Moreover, since the probes 52 are worn down, it is necessary to replace the probes 52 periodically. In addition, the measurement can not be carried out in a vibrating state. Furthermore, this method is limited by a prerequisite of providing a stage which is to be used exclusively for suction. Therefore, if the available space is limited, it is difficult to mount the stage.
An alternative method measures the resistivity of a semiconductor material in a non-contact manner without bringing a metal probe or the like into contact with a sample by pressures. According to this method, as illustrated in FIGS. 19 and 20, with the use of a coil 54 connected to a high frequency oscillating circuit 53, an eddy current is created in a metal layer on a substrate 55 to be measured, i.e., on a glass or wafer, by inductive coupling. At this time, the created eddy current produces Joule's heat and is lost. Since there is a positive correlation between the absorption of high frequency power in a semiconductor wafer and the conductivity, the conductivity (the inverse number of the resistivity) of the semiconductor material can be obtained from the positive correlation in a non-contact manner. Compared with the four point probe method, the above-mentioned measuring method using an eddy current has an advantage because it can evaluate the sheet resistance of a metal layer in a non-contact manner without contaminating the wafer nor applying pressures to the wafer in the final processing step.
However, in a conventional managing method, the measurements of the sheet resistance are carried out randomly by the four point probe method, and the film deposition conditions are controlled according to the results of the measurements. Therefore, if an unexpected defect occurs or the film thickness varies, the substrates having films deposited before the detection of such a defect or a variation in the film thickness are wasted. As a result, the yield is lowered, and the cost is increased.
Another measuring method using an eddy current is disclosed in Japanese laid-open patent publication "Tokukaihei" No. 5-21382 (published on Jan. 29, 1993). In this method, an eddy current is created in a sputtered film in a non-contact manner, and the line of magnetic force resulting from the eddy current is detected and the sheet resistance is calculated.
FIG. 21 shows schematically a cross section of the structure disclosed in the above Japanese patent publication "Tokukaihei" No. 5-21382. Here, a load-lock chamber 57 is formed next to a gate valve 56 of a sputtering room, a transporter 59 for transporting a substrate 58 is provided in the load-lock chamber 57, and a system for controlling the sheet resistance of a sputtered film to be deposited on the substrate 58 is produced in this sputtering device. More specifically, in the load-lock chamber 57, the sheet resistance of the sputtered film is measured using a time in which the temperature of the substrate 58 having the sputtered film deposited thereon is decreased. With this system, the sheet resistance can be measured without lowering the tact. Incidentally, the system includes a resistance measuring device 60 for measuring the resistance of the sputtered film, a gas controller 61 for transmitting a reaction gas flow rate control signal according to the resistance measured, and muss flow controllers 62 for adjusting the amount of the reaction gas in accordance with the control signal transmitted.
An eddy current sensor (resistance measuring device 60) applies a high frequency current to an incorporated coil, and detects the changes in magnetic field and electric field according to the distance from a conductor (sputtered film on the substrate 58) as the change in the inductance of the coil. Hence, the eddy current sensor is electromagnetically sensitive to an extreme extent. Therefore, it is not preferred to install a heater in the proximity of the eddy current sensor as the heater affects the sensor electromagnetically. However, according to the structure described in the above-mentioned publication, the temperature of the substrate 58 after the deposition of the film is high, and the eddy current resistance measuring device 60 is substantially affected by the temperature as shown in FIGS. 22 and 23 due to the influences of the expansion of the coil and the temperature dependence of the sheet resistance. As a result, large variations occur in the sheet resistance.
In order to avoid such variations, it is necessary to maintain a uniform temperature difference between the resistance measuring device 60 and the substrate 58 by performing heating with a heater while preventing a drift of temperature. In other words, in order to achieve high precision by preventing the vicious effects of the temperature variations, it is necessary to actively control the temperatures of the eddy current sensor and the substrate. In addition, it is necessary to satisfy a controversial demand that the original operation and function of the eddy current sensor must be maintained. In order to satisfy the demand, it is necessary to provide temperature control sensor means for maintaining a uniform temperature difference, and a temperature sensor for detecting the temperatures. Consequently, the cost is increased.
Furthermore, in the eddy current sensor, if the gap between the coil and the sputtered film varies, an output of the sensor varies according to the gap. Therefore, the sheet resistance can not be measured accurately. For instance, if the gap is increased, the magnetic flux decreases and the eddy current flowing in the sputtered film decreases. It is thus necessary to maintain a uniform gap between the coil and the sputtered film. Hence, the above-mentioned eddy current sensor creates an eddy current by applying a high frequency magnetic field to an object to be detected (sputtered film) so as to vary the impedance of the sensor coil according to the distance between the eddy current sensor and the object, detects the change in the oscillation state with an amplifier, and measures the distance. However, when the object to be measured is not parallel to the sensor, particularly when the object is vibrated or warped, the distance can not be measured accurately. In actual, it is often the case that the substrate 58 itself is warped by heat deformation. In such a case, if the non-contact type eddy current sensor is used, since a variation in the measuring height occurs, it is difficult to perform accurate measurements.
Besides, if the resistance measuring device 60 is mounted in the load-lock chamber 57, the maintenance efficiency is worse, and the working efficiency is lowered.